Speech synthesizers are well known in the art and are described in various U.S. Patents. References to speech synthesis include the following:
Three-chip System Synthesizes Human Speech, by Richard Wiggins and Larry Brantingham, Electronics, Aug. 31, 1978. This reference describes an early speech synthesizer employing linear predicitive coding (LPC) and using periodic impulses for voiced excitation and white noise for unvoiced excitation.
Design case history: Speak & Speel learns to talk, IEEE Spectrum, February, 1982, pp 45-49.
Products that talk, by Eric J. Lerner, IEEE Spectrum, July 1982, pp 32-37.
Realism in synthetic speech, by Gadi Kaplan and Eric J. Lerner, IEEE Spectrum, April, 1985, pp 32-37.
Code-Excited Linear Prediction (CELP): High Quality Speech at Very Low Bit Rates, by Manfred R. Schroeder and Bishnu S. Atal, ICASSP, 1985 IEEE, pp 25.1.1.-25.1.4. This reference illustrates the use of short and long delay predictors in voice transmission using codebook innovation sequences.
The most popular speech synthesizers, such as those manufactured and sold widely by Texas Instruments and described in the above article by Wiggins et al, employ a Linear Predictive Code (LPC) filter which operates on excitation functions which are either a series of pulses having varying spacing therebetween or white noise. Less popular speech synthesizers, such as those manufactured by Philips, employ a formant filter which operates on the same excitation functions as LPC synthesizers.